Chokes are used in flow lines to create a restriction in the flow. In many applications of chokes, the high velocity flow or the entrained moisture or particles are particularly damaging to the tip of the choke valve member. In such applications, the tip is made of an erosion resistant material, such as tungsten carbide. Rather than make the whole of the needle or valve member out of the erosion resistant material, the tips are made to be replaced.
Prior to the present invention, such tips have been silver brazed onto the needle stem. Few are qualified to perform this silver brazing so that it is difficult to replace a tip and have it properly installed.
Threading of the tungsten carbide tip would be advantageous except that forming threads in tungsten carbide is an expensive operation.
U.S. Pat. No. 852,644 discloses a refractory stopper which includes internal threads and a nut embedded in the stopper through which the threads extend so that the stopper can be threaded onto the stem.
U.S. Pat. No. 1,499,433 discloses a throttle valve in which the tip is attached to the stem with a ring positioned in a groove in the stem and a plug threaded into the tip and engaging the ring to thereby secure the tip to the stem.
U.S. Pat. No. 1,831,713 discloses a choke or adjustable flow bean in which the tip is threaded onto the stem into abutment with disc which is secured to the stem with a set screw being used to prevent the unthreading of the tip from the head.
U.S. Pat. No. 1,911,905 discloses a choke or adjustable flow bean in which the tip is secured to the stem by a pin extending through the stem and through the outer portion of the tip which surrounds the stem and through the stem.
U.S. Pat. No. 3,262,673 discloses another stopper structure which includes a split sleeve positioned in a stem groove and having external threads which are adapted to engage within internal threads in the stem opening in the stopper.
U.S. Pat. No. 3,693,732 discloses a well control choke in which the stem is surrounded by a sleeve which receives a plug in an enlarged recess in its outer end which extends beyond the end of the stem and is held in position by the threading of the plug onto the end of the stem.
U.S. Pat. No. 3,761,053 discloses a needle valve in which the needle is replaceable and is provided with a spherical end which is received within a slot in the stem or is secured by a collar retaining it against the end of the stem.
U.S. Pat. No. 4,337,920 discloses another method of securing a wear-resistant, sleeve-type valve member to its stem in a choke which is subject to high temperatures and extreme erosion conditions. This includes the utilization of annular springs positioned around the stem and within the wear-resistant sleeve.